The long term research objective in this laboratory is to elucidate cellular and molecular mechanisms of development in leech embryos (phylum Annelida), for comparison with other phyla. Such comparisions allow us to determine which features of developmental mechanisms discovered in organisms such as Drosophila are more likely to be operating in human development. Of particular interest are the morphogenetic processes orchestrating the assembly of segmental structures. In both chordates and annelids, segmental structures arise from sheets of cells generated by a posterior growth zone comprising embryonic stem cells. In leech, these cells are few in number, individually identifiable and accessible for experimental manipulation. Thus, basic research on the experimentally accessible leech embryo can contribute to the understanding and eventual amelioration of genetic and environmentally induced developmental disorders. In leech, segmental mesoderm and ectoderm arise in a anterior-to- posterior progression as the progeny of five bilaterally paired columns of segmental founder cells (m, n, o, p and q primary blast cells) in the germinal plate of the embryo. The five columns of blast cells on each side arise from uniquely identifiable M, N and Q teloblasts, plus a pair of equipotent O/P teloblasts. In both the N and Q lineages, primary blast cells adopt 2 distinct fates in exact alternation, and two primary blast cells (one cell of each type) contribute progeny to each segment. In each of the other three lineages (M, O and P), all blast cells follow roughly identical fates and each blast cell contributes a full segmental complement of progeny. The experimental goal for the proposed project period is to investigate the hypothesis that homologs of Drosophila pair rule genes are essential for specifying the stem cell divisions by which teloblasts generate segmental found cells in leech, and for determining distinct identifies of alternate blast cells in the N and Q lineages.